Pistons are used in caliper disc brakes to forcefully engage brake pad assemblies or pads against a brake rotor in order to stop the wheels of automotive vehicles. Such pistons are generally comprised of a hollow piston body slidably mounted in the bore of a brake caliper that is connected to an actuator such as a source of pressurized brake fluid. When the piston body is acted on by pressurized brake fluid, it forcefully slides in the caliper bore toward the brake pad. The distal end of the piston body engages the brake pad and pushes it against the brake rotor in order to slow or stop the wheel of a vehicle.
Such forceful engagement and the resulting high frictional forces generate considerable heat which can damage the disc brake mechanism over time by overheating and deteriorating the seals and thermally fatiguing the caliper body and components. Additionally, heat generated within the brake pad and conducted through the piston body can cause the brake fluid behind the piston body to boil, thereby compromising the braking action and causing an unsafe condition for the driver.
To solve these problems, heat insulated piston cap assemblies have been developed which comprise a hollow piston body having a closed end and an open end, and a piston cap having a brake pad engaging portion, and a retention portion mounted to the open end of the piston body. A heat insulating ring is provided between the interface of the piston body and the retention portion of the piston cap to prevent heat from being conducted to the brake fluid via the piston body. While such heat insulating rings are effective for this purpose, they are formed from relatively expensive materials such as a ceramic filled polymer or other materials which can significantly increase the cost of the piston cap assembly.